Field of the Disclosure
Embodiments of the present application relates to the art of a control system for a hybrid vehicle in which a prime mover includes an engine and two motors, and especially to a control system configured to startup an engine during propelling a vehicle by a motor while stopping an engine.
Discussion of the Related Art
A hybrid vehicle can be powered only by a motor when a required driving force is small. In the hybrid vehicle of this kind, an engine is started when an accelerator pedal is depressed deeply to increase a drive force during propelling the vehicle by the motor. In this situation, the engine is started by one of the motors as a prime mover. JP-A-2013-56600 describes an engine start control device configured to start an engine by a motor being used to power a vehicle.
According to the teachings of JP-A-2013-56600, the engine is connected to a carrier of a planetary gear unit, and a reverse rotation of the carrier is prevented by a one-way clutch. A first motor is connected to a sun gear of the planetary gear unit, and a second motor is connected to a ring gear connected to an output member to apply torque to the output member. In a nomographic diagram of this planetary gear unit, the carrier connected to the engine is situated between the sun gear connected to the engine and the ring gear to which the torque of the second motor is applied. Since the reverse rotation of the carrier can be stopped by the one-way clutch, the ring gear or the output member connected to thereto is rotated in the forward direction by applying a reverse torque of the first motor to the sun gear. In this situation, the vehicle can be propelled in a dual-motor mode in which the vehicle powered by the first motor and the second motor by rotating the second motor in the forward direction.
When starting the engine in the hybrid vehicle taught by JP-A-2013-56600 during propulsion in the dual-motor mode, the control device brings a brake device into engagement to deliver the forward torque to the carrier while controlling the first motor to generate the reverse torque. Specifically, the brake device is arranged in such a manner as to be situated between the sun gear and the carrier in the nomographic diagram of the planetary gear unit. According to the teachings of JP-A-2013-56600, therefore, the brake device is allowed to serve as a reaction element instead of the one-way clutch receiving the reverse torque of the first motor to startup the engine connected to the carrier by the forward torque.
According to the teachings of JP-A-2013-56600, the first motor is rotated in the same direction during propulsion in the dual-motor mode and when starting the engine. That is, a drive force generated by the first motor will not be dropped to zero even temporarily. However, as indicated in nomographic diagrams of JP-A-2013-56600, torque multiplication of a case in which the brake device establishes a reaction torque against the reverse torque of the first motor may be reduced in comparison with torque multiplication of a case in which the one-way clutch establishes a reaction torque against the reverse torque of the first motor. As a result, temporal drop in the drive force and shocks may be caused. Specifically, the torque multiplication is a ratio of an output torque of the first motor to a torque applied to the ring gear in the forward direction. In order to apply the torque to the carrier in the forward direction, the brake device has to be situated between the sun gear and the ring gear in the nomographic diagram. Consequently, a distance between the sun gear and the brake becomes shorter than that between the sun gear and the carrier in the nomographic diagram. For this reason, the torque multiplication may be reduced during applying the output torque of the first motor to the ring gear thereby causing a drop in the drive torque generated by the first motor.